1. Field of the Invention
The present invention relates to an optical scanning device mounted on an image forming apparatus such as a digital copier or laser printer using an electrophotographic process, and also relates to an image forming apparatus including such an optical scanning device.
2. Description of the Related Art
An optical scanning device is widely known in connection with an image forming apparatus such as an optical printer, optical plotter, and digital copier, in which a light beam from a light source is deflected by an optical deflection and scanning device such as a rotary polygonal mirror, the deflected light beam is directed by a scanning and imaging system such as an fθ lens to be focused on a scanning target surface to form an optical spot thereon, and the scanning target surface is optically scanned by this optical spot.
The image forming apparatus using the optical scanning device, in general, performs image forming processes including an image writing process to write an image by an optical scan. The quality of the formed image is dependent on the scanning characteristics, and the quality of the optical scan depends on the scanning characteristics in a main scanning direction and a sub-scanning direction.
One of the scanning characteristics in the main scanning direction includes a constant velocity of the optical beam scanning. For example, if a rotary polygonal mirror is used as an optical deflection scanning means, the deflection of the optical beam is performed at equal angular velocities. Then, to achieve a constant velocity during optical scanning, a scanning imaging optical system having fθ characteristic is used. However, in light of other characteristics required for the scanning imaging optical system, a perfect fθ characteristic cannot be attained easily. Thus, in an actual optical scanning, the optical scan is never performed at constant velocity, and the constant velocity as one of scan characteristics includes a variation from an ideal constant velocity scanning.
The scan characteristic in a sub-scanning direction includes skewing of the scan line and tilting of the scan line. Ideally, however, the scan line is a movement locus of an optical spot and should be a straight line. Thus, the optical scanning device is so designed that the scan line becomes a straight line. However, in actuality, the scan line tends to skew due to errors in processing and assembly.
In addition, in a case in which an imaging mirror is used as a scanning imaging optical system and the deflection light beam is configured to have an angle between the incident direction to the imaging mirror and the reflection direction therefrom in the sub-scanning direction, in principle the skew in the scan line occurs. Even when the scanning imaging optical system is configured as a lens system, the scan line skew inevitably occurs in a so-called multi-beam scanning method in which the scan-target surface is scanned optically with a plurality of discrete optical spots in the sub-scanning direction.
The scan line tilt is a phenomenon in which the scan lines do not correctly intersect in the sub-scanning direction and is a type of scan line skew. Accordingly, unless specified otherwise, in the following description the scan line tilt is included in the scan line skew.
If the constant velocity of the optical scanning is not perfect, the formed image is distorted in the main scanning direction. The scan line skew causes the formed image to have distortion in the sub-scanning direction. In a case of a monochrome image, if such an image is written and formed by a single optical scanning device, if scan line skew and imperfection of the constant velocity (i.e., the deviation from the ideal constant velocity scanning) are regulated and kept within a certain range, the formed image does not include a distortion observable by the naked eye; however, such distortion preferably does not exist in the formed image in the first place.
Conventionally, the colors magenta, cyan, yellow, and black are used as color components, and respectively formed images are overlaid one atop another to synthesize a complete color image.
In such color image formation, there is a so-called tandem method, in which each color component image is formed on a corresponding one of multiple different photoreceptors by a corresponding one of multiple different optical scanning devices. Specifically, as color image forming devices, a plurality of photoreceptors are provided such as, for example, a photoreceptor for yellow (Y), a photoreceptor for magenta (M), a photoreceptor for cyan (C), and a photoreceptor for black (K). A laser beam is fired, based on image information of each color, by the optical scanning device corresponding to each photoreceptor, and a latent image is formed on the photoreceptor for each color and is developed by developing means to form a toner image. The toner images thus formed on respective photoreceptors are sequentially overlaid on a single transfer medium, thereby forming a desired color image.
In the aforementioned image forming method, if a degree of scan line skew and tilt of each optical scanning device is different from each other, then even though the scan line skew in each of the optical scanning device is corrected, the superimposed image formed on the single transfer medium may include a type of image abnormality called a color shift, marring the quality of the image. If this color shift occurs, the hue changes and color variation occurs, thereby failing to obtain the desired image. The reasons of the color shift vary from optical scan position errors due to the difference in the temperature of the respective optical scanning devices, to the skew and tilt of the scan line of the respective optical scanning devices.
To correct such color shifting errors, conventionally, registration error patch patterns are formed on the transfer medium for color shift detection at a time of power up or at a predetermined print cycle, and are detected by a charge couple device (CCD) to thus detect a shift amount relative to reference colors. Such CCDs are disposed at both ends of the transfer medium in the image forming area. By detecting the shift amount at two points, the scan line skew in each of the optical scanning devices is detected.
However, because color shift correction is performed mechanically, the correction cycle takes a lot of time and cannot be performed during an ordinary print cycle. In actuality, this correction cycle takes a few minutes. Therefore, if this correction cycle is performed during the print cycle, the user will have to wait until the end of the correction cycle.
Further, recently, the imaging optical system of the optical scanning device has generally come to adopt a specific surface such as an aspheric surface to improve scan characteristics. As a result, an imaging system employing resinous materials has been popularized for easier manufacturability and lower cost.
Optical characteristics of the imaging system formed of resinous materials tend to be affected by changes in temperature and humidity. Such change in the optical characteristics may change the degree of skew of the scan line and vary the constant velocity. Consequently, if for example several tens of color images need to be formed continuously, continuous operation of the image forming apparatus increases the temperature inside the apparatus, thereby changing the optical characteristics of the imaging optical system and thus also gradually changing the skew degree or the constant velocity of the writing scan line performed by each optical scanning device gradually. As a result, the color image formed initially and that formed at the end of the job can be very different from each other due to the color shifting phenomenon.
In particular, the scanning imaging lens such as an fθ lens, which is representative of scanning imaging optical systems, is in general formed such that part of the lens into which the deflected light beam is not incident in the sub-scanning direction is cut and a reed-shaped lens elongated in the main scanning direction is formed. When the scanning imaging lens is formed of a plurality of lenses, the farther the lens is positioned away from the light deflection scanning means, the larger the lens length in the main scanning direction becomes. Then, a so-called elongated lens having 10-odd to more than 20 centimeters is formed.
Such an elongated lens is formed in general using resinous materials, and therefore, if the temperature distribution inside the lens becomes uneven due to the external temperature change, the lens tends to have a bow and to be arched in the sub-scanning direction. Such a bow in the elongated lens causes the scan line skew described above. If the bow is particularly acute, the scan line skew will also be acute.
As a means to correct this scan line skew, an electro-optic element has been developed recently, in which voltage applied to a crystal of the element is controlled so that the refractive index of the crystal is changed to deflect the light. Compared to the optical scanning device used in the image forming apparatus, the light deflection amount is small, but the applied voltage can vary the refractive index at high speed. For this reason, the electro-optic element is optimal for correcting the scan line skew.
JP-3164002-B discloses a technique related to a means to correct scan line skew amount using an electro-optic element to cope with the aforementioned problems, and proposes a color image forming apparatus capable of correcting the scan line skew or the color shift amount by using the electro-optic element to deflect the transmitted optical beam by an amount corresponding to the applied voltage.
However, the invention disclosed in JP-3164002-B above could not perform high-speed shift amount detection and high-speed correction due to the time needed to detect a shift amount, the time for cleaning, and the like.
JP-3450402-B discloses a technique to correct a color shift by forming a color shift detection pattern on the transfer belt between conveyed sheets when any environmental change in the temperature or humidity occurs which may cause scan line skew. However, this method could not perform correction of the scan-line skew and tilt.
JP-3833542-B, JP-4170637-B, JP-4369658-B, JP-2006-133287-A, and JP-2006-133288-A disclose a method in which liquid crystal deflecting elements are arranged over an entire area of the scan line, and without performing pattern formation to the transfer belt to detect a shifting amount, by using effectively the light from the liquid crystal elements the scan line skew amount to be written on the photoreceptor is detected, thereby correcting the skew and tilt in the scan line and fluctuation in the constant velocity of the light beam. This method, however, may drastically increase manufacturing costs due to the use of an elongated, large liquid crystal element. In addition, the liquid crystal element has a disadvantage in that, because substances with different expansion coefficients are used in combination, spherical aberration occurs due to the effect of temperature change, and the beam characteristics on the photoreceptor tend to vary from the desired characteristics. The responsiveness of the liquid crystal element (of approximately 1 kHz) to the polygon motor frequently used as an optical scanning device having a scanning frequency of approximately 1 kHz is not good, and real time positional correction to each scan line is not possible.